1. Field of the Invention
Naturally occurring liquid hydrocarbons, such as crude petroleum, are typically a complex composition of many individual organic chemicals of differing molecular length, weight and form. Some of these organic chemicals are quite volatile, with some of the volatile organic chemicals of special environmental concern. For instance benzene, toluene, ethyl benzene and xylene are strictly regulated in the United States. As the existence of personnel and environmental hazards, the cost of mitigating said hazards, and other important characteristics of a liquid hydrocarbon depend fundamentally on whether a liquid hydrocarbon emits vapors at a given pressure and temperature, the accurate determination of the vapor pressure of a liquid is of key importance. Of equal importance is the capability to modify the measured vapor pressure to bring a liquid hydrocarbon within desired or mandated vapor pressure, and possibly vapor composition, conditions.
In addition to compliance with regulatory requirements regarding vapor characteristics, or achieving specified vapor characteristics for processing purposes (as in refinery applications), the ability to do so in an economically optimum manner is important. Such regulatory and operational requirements may be achieved by controlled blending of multiple liquid hydrocarbon streams of different vapor characteristics and unit costs, to yield a blended liquid representing the optimum combination of vapor characteristics and unit costs.
The present invention relates generally to apparatus and methods for continuous determination of the amount and volatility of hydrocarbon compounds contained in a liquid hydrocarbon stream. The present invention also relates to apparatus and method for continuous, real-time modification of the vapor characteristics of a liquid hydrocarbon stream, responsive to measured characteristics of the stream, so that excessive amounts of undesirable VOCs ("Volatile Organic Compounds") may be removed from the liquid before exposure to atmospheric conditions. In addition, oil shrinkage (effluent volume as compared to influent volume) and gas/oil ratio (GOR) may be determined with the apparatus. Therefore, the present invention relates to the field of continuous direct analysis of an influent stream of liquid hydrocarbons and, as required, modification (responsive to said analysis) of the composition of said stream, for production of an effluent stream of liquid hydrocarbons having desirable vapor characteristics. In addition, means for identifying the volatile components of the liquid hydrocarbon stream may be provided.
2. Description of the Related Art
Many liquid hydrocarbons, particularly those naturally occurring such as liquid crude oil, are a complex admixture of many hydrocarbon compounds of differing molecular length, weight and/or molecular geometry. Those hydrocarbon compounds typically vary, for instance in naturally occurring crude oil, from compounds having a low molecular weight, such as methane (having one carbon atom) to compounds having many carbon atoms and a relatively high molecular weight, such as various asphalt molecules. Those compounds in the lower range of molecular weights are commonly referred to as the "light ends", while those in the upper range of molecular weights are commonly called the "heavy ends". While in pure form the vapor pressure curve (a pressure versus temperature curve defining a line of pressures and corresponding temperatures at which liquid to gas and gas to liquid transfer is in equilibrium) of each of these individual compounds is generally well known, in field application the vapor pressure characteristics of a complex admixture are not readily predictable because of two factors. First, in complex admixture, many of the individual components interact chemically and physically, modifying what might be expected to be a particular compound's partial pressure contribution to the total vapor pressure of the complex admixture. Secondly, precise analysis of the exact chemical and physical structure and amount of each component in a complex admixture is generally cost prohibitive, time consuming, includes various inaccuracies (as separation of the components for detailed analysis can change the components and their interaction with each other) and is of limited temporal utility. Further, it is ordinarily not necessary to know the precise composition and amount of every component of crude liquid petroleum at every point in time to establish the value or utility of the liquid petroleum.
However, because of exigent environmental and safety concerns, there is a need for greater information about and control of the vapor characteristics of liquid hydrocarbons. Due to the massive quantities of crude oil production worldwide significant environmental concerns have arisen, and will likely continue to arise, from emissions of VOC vapors therefrom. Of special environmental concern are certain compounds believed to be hazardous, commonly referred to as "BTEX" (benzene, toluene, ethyl benzene and xylene). For instance, the 1990 Clean Air Amendments establish as major sources of air pollution any facility which emits more than a specified annual quantity of any one or a combination of hazardous pollutants and require control of such emissions.
Significant other concerns also arise from the volatile components of liquid hydrocarbons in their storage and processing. All of said VOCs are at least inflammable or explosive in certain concentrations, posing acute safety hazards in enclosed conditions. In pumping operations gas bubbles within the liquid stream can occur under certain pressure and temperature conditions, affecting pump operation and system safety. The volatile components of crude liquid petroleum can affect its value in several respects; such components indicate how much liquid "shrinkage" may occur in storage, what kind of storage facility may be required, and the expenses that may have to be incurred to prevent excessive emissions. Additionally, the chemical and/or physical characteristics of vapor quantity and content frequently affect the usability of the liquid hydrocarbon admixture for certain purposes, the nature and expense of processing which may be required to produce desired products, the risks to personnel and to the environment attendant to handling or use of the admixture, and many similar concerns. For these reasons, it is desired to have a means for continuously, precisely, and accurately analyzing through direct measurement and, if necessary, modifying the quantity and/or character of the volatile components of a liquid hydrocarbon flowstream, under dynamic conditions, the modifications being performed responsive to said analysis.
The necessary analysis poses several problems. For example, current empirical methods of vapor pressure determination are often substantially in error when compared to more accurate measurements done under controlled, static conditions in a laboratory, such as in static pressure-volume-temperature (PVT) cells. One such empirical method is the Reid Vapor Pressure Test according to ASTM D-323-90. The Reid method involves collecting a sample of the liquid hydrocarbon at atmospheric pressure, chilling the sample, combining the liquid hydrocarbon sample with an air volume at 100.degree. F., then agitating the combined air volume and liquid hydrocarbon sample until the pressure equalizes. The resulting pressure is then reported as the Reid Vapor Pressure.
It has long been recognized that Reid Vapor Pressure values may be greatly in error. Efforts to improve the accuracy of the Reid Vapor Pressure test have generated "correction" procedures such as that outlined in API 2517. The API 2517 procedure utilizes a nomograph in an effort to generate a "true" vapor pressure from the Reid vapor pressure value and the actual stock tank liquid hydrocarbon temperature. However, laboratory tests have shown that even the API 2517 value is often in error.
Static laboratory-type PVT measurements are not practical for field testing of large liquid hydrocarbon volumes due to the lengthy test time required and the relatively small sample volume tested. Further, such static PVT cell apparatus are of a relatively fragile nature, not readily compatible with field usage.
Spicar, U.S. Pat. No. 5,339,672 (Aug. 23, 1994) discloses an apparatus for monitoring hydrogen gas present in electrical transformer oil. A cell has a glass filter within, disposed a distance from the top of the cell, the glass filter having small pores therethrough. Transformer oil is flowed into the upper oil compartment between the glass filter and the top of the cell. Oil flows through the glass filter under pressure and then to the bottom of the cell where a lower gas and liquid space are formed. Lower liquid level is controlled by a liquid effluent pump. Gas from the gas space (driven by an induced temperature differential) flows through a hot wire gas detector which detects the concentration of hydrogen in the gas. No means is disclosed for determining the quantity and composition of organic vapors at controlled temperatures and pressures and modifying the vapor characteristics of the liquid as desired.
Fleming, U.S. Pat. No. 5,222,032 (Jun. 22, 1993) is directed to apparatus and method for determining the amount of organic chemicals, such as carbon tetrachloride, in a liquid wastestream, such as water. Wastewater is injected into a vessel at a sufficient velocity to create bubbles and turbulence in a vessel. The turbulence is said to enhance breakout of gases entrained in the liquid. A stripper gas may also be injected into the liquid to enhance gas breakout. The quantity of undesirable gases is then measured as a function of time, determining the response time of the system. No means, responsive to effluent characteristics, is disclosed for control of temperature and pressure of the vessel, or for blending of multiple influent streams of differing characteristics.
Other apparatus, such as those disclosed in Kanba, et al, U.S. Pat. No. 5,062,292 (Nov. 5, 1991) and Baughman, et al, U.S. Pat. No. 5,191,786 (Mar. 9, 1993), determine gas dissolved in batch oil samples by injecting "stripper" gas into the oil (which may be diluted) and then analyzing the combined stripper gas/extracted gas sample.
Henderson, U.S. Pat. No. 5,499,531 (Mar. 19, 1996) discloses a system and method for calculating the composition of a liquid hydrocarbon using an iterative mathematical algorithm. From the calculated composition of the liquid hydrocarbon, the partial pressure of each hydrocarbon compound therein may be calculated and aggregated to produce a calculated total vapor pressure. Henderson addresses direct liquid vapor pressure determination only as a "check" method on the iterative liquid composition calculation/partial pressure method. The apparatus and method allegedly invented by Henderson and set forth in U.S. Pat. No. 5,499,531 discloses no pressure control means by which separation chamber pressure may be sustained at sub-atmospheric values as well as above atmospheric values for continuous liquid vapor characteristic modification and direct measurement thereof. In Henderson, temperature control of the liquid influent is limited to maintaining temperature only high enough to evolve some vapor therefrom for gas composition analysis; further, said temperature control is performed only by means external to the vessel, and no diffusion means within the vessel is disclosed. Further, Henderson discloses no means by which multiple liquid hydrocarbon streams may be blended to yield a composite liquid hydrocarbon influent having desired vapor characteristics.
None of these patents disclose the present invention. The related art shows no apparatus or method which provides a direct, continuous measurement of the vapor characteristics of a hydrocarbon stream at pressures controlled below or above atmospheric pressure, at a selected temperature. Further, no apparatus is shown which is capable of modifying the vapor characteristics of a liquid hydrocarbon influent stream by pressure and temperature control and blending, responsive to effluent characteristics, to produce a liquid hydrocarbon effluent having desired vapor characteristics at pressures below and above atmospheric. The apparatus of the present invention provides an accurate, field-compatible means for measurement and modification of the vapor characteristics of a hydrocarbon stream.
A general object of the present invention is to provide improved apparatus and method for measuring the vapor characteristics of a stream of liquid hydrocarbons, such as liquid petroleum. Another general object of the present invention is to provide improved apparatus and method to modify the vapor characteristics of an influent liquid hydrocarbon stream so as to produce an effluent liquid hydrocarbon stream having desired vapor characteristics.
With more particularity, other objects of the present invention are to provide transportable apparatus adaptable to field use; to provide methods which are simple to follow; to provide apparatus and methods for direct, continuous measurement of total vapor pressure of a flowing stream of liquid hydrocarbons; to provide means for measurement of the vapor pressure of hydrocarbons with increased accuracy; to provide apparatus and methods for modification of the vapor characteristics of a hydrocarbon stream; to provide apparatus and methods having a plurality of means for modification and control of the vapor pressure characteristics of a hydrocarbon stream; to provide apparatus and methods for determining liquid hydrocarbon shrinkage and GOR; and to provide apparatus and methods wherein blending of influent streams and/or temperature and/or pressure at which liquid/gas separation occurs may be varied, in real time, responsive to measurement of effluent characteristics, to produce a liquid effluent of desired characteristics. Yet another object is to perform such modifications with an optimum expenditure of energy, by using a combination of pressure control, temperature control, and blending.